6.1 Overview of an Observing Program

6.1.1 General Observer (GO) Programs

6.1.2 Snapshot Programs

6.1.1 General Observer (GO) Programs

Definitions (HST Orbits, Visibility Periods, and Visits)

Components of a Visit

Preparing a Phase I Program

Definitions (HST Orbits, Visibility Periods, and Visits)

HST GO observing time is counted in terms of orbits. Each 96-minute orbit contains a certain amount of useful time when the target can be observed, called the visibility period. The length and timing of the visibility period depends on the declination of the target and on whether there are any special scheduling constraints. Orbits are grouped into larger units called visits; a visit is a series of one or more exposures on a target, including overheads, that will execute in one or more consecutive orbits.

Components of a Visit

The orbits in a visit generally contain the following components:

•

Guide star acquisition and re-acquisition of a target is required to ensure that HST can maintain adequate pointing during each orbit. Guide star acquisition is needed for the first orbit in a visit, and re-acquisition is needed for subsequent orbits in that same visit. See Section 3.2 for details on telescope guiding.

•	Target acquisition is needed if the target must be placed in an instrument aperture. Imaging observations (unless they are coronagraphic) generally do not require a target acquisition. See Section 5.2 for details on target acquisition strategies.

•	The science exposures.

•	Instrument overheads (e.g., the time required to set up the instrument and read out the data).

•	Telescope repositioning overheads for small angle maneuvers such as pattern dithers and offsets.

•	Special calibration observations which may be required if the accuracy provided by the standard calibrations is inadequate for the goals of the project (see Section 4.3 of the Call for Proposals).

Preparing a Phase I Program

Use the following steps to calculate the resources required for a GO program:

1.	Define the observations (instrument set-up, number of exposures, exposure times, etc.) for each target. Use the instrument handbooks at the HST Instruments Web page and the Exposure Time Calculator (ETC) tools as primary resources.

2.	Group observations into separate visits following the guidelines in Section 6.2.

3.	Determine the visibility period of each target in the proposal (described in Section 6.3). Programs requesting 100 or more orbits in a single cycle must use the visibility periods listed in Table 6.1. For LARGE programs, these visibility periods will be enforced for approved programs in Phase II.

4.	Compute the times required for guide star acquisitions, target acquisitions, instrument overheads, and telescope repositioning overheads (described in Section 6.4 and Appendix A). See Section 3.2.3 of the Call for Proposals.

5.	Lay out all the exposure and overhead times for the program into visits (described in Section 6.5) and add up the number of orbits from each visit to obtain the total orbit request. Each visit must consist of an integer number of orbits. Partial orbits are not granted.

6.1.2 Snapshot Programs

In a Phase I Snapshot proposal, the PI specifies a requested number of targets, rather than a requested number of orbits. The exposure times and overhead times for Snapshot observations are calculated in similar fashion as for GO observations. The observations for a Snapshot target, including overheads, must not exceed 45 minutes. See Section 3.3 of the Call for Proposals for detailed policies and procedures for Snapshot observations.